1. Field of the Invention
The present invention relates to an image forming apparatus used to express gradation by means of screen-processing an input image, in particular, to an image forming apparatus capable of suppressing the moiré effect.
2. Description of the Related Art
There is a case when an image with multiple gradations is applied to an output apparatus capable of handling a number of gradations less than the number of the gradations of the image itself. In a typical output apparatus of the prior art, the output is treated with a screen processing using a plurality of dither patterns so that a number of gradations higher than that of the gradation of each pixel can be obtained by treating a plurality of pixels as a unit.
In a case where a quasi-halftone is reproduced to output a dot matrix document using a dither pattern with a specific screen angle such as a dot dither, a problem of the moiré effect can occur depending on the relation between the screen angle of the document image and the screen angle of the dither pattern.
When the line number of the dots of the document image is close to that of the dither of the output apparatus, and also the screen angle of the document and that of the dither is almost the same, the moiré effect of a low frequency is likely to occur.
Generally speaking, the line number of the dots and the screen angle of the document are not constant. For example, the document has a specific screen angle for each color of cyan, magenta, and yellow. Therefore, the moiré effect can occur depending on a document when a quasi-halftone is reproduced using a specific dither pattern.
As a means of avoiding the moiré effect, it is conceivable to increase the line number of a dot matrix document to a degree that there is practically no problem. For implementing said idea, however, the output apparatus is required to have an output capability of a higher degree of resolution to secure gradation and a control capability sufficient for area modulation and intensity modulation below one dot level. In other words, the output apparatus must have an extremely high performance capability, which makes it impractical from the cost standpoint.
Japanese Patent Unexamined Publication, JP-A-08-23444 discloses a method of avoiding the moiré effect without recourse to the use of a high performance output apparatus. The method consists of (a) converting the image data before and after the screen processing into frequency planes respectively; (b) assuming that a moiré effect occurred when a low frequency peak appears that did not exist on the frequency plane of the image before the screen processing; and (c) selecting an optimum screen size. This method does not require a high performance output apparatus, so that it is relatively easy to market it.
In essence, said method is to detect the moiré effect based on the image after the screen processing, and to change the screen size if it is determined that the moiré effect has occurred. As a result, resolution lowers if the screen size is larger than the original size, and gradation deteriorates if the screen size is smaller.
Generally, the screen size of an output apparatus is set to a size that optimizes the balance between resolution and gradation considering performance of the output apparatus. Therefore, the output image quality will be adversely affected if either resolution or gradation is affected as a result of the screen size change intended to suppress the moiré generation.